Proposal: add exec runner agent autoscaling to drone-autoscaler

We use drone-runner-exec for our builds and will like to add exec-runner autoscaling support to autoscaler.

An initial list of features:

  • ability to disable installation of docker runner.
  • ability to install drone-runner-exec binary on agents.
  • ability to create config.txt from autoscaler environment variables.
  • pinger for exec agents.

Draft PR: